Documentation, Reference Materials and Tutorials for Imagely
Thumbnail Options settings are available when creating or adding a gallery directly from your post or page.
They’re also available under Imagely > Settings > Image Options.
When clicking into to page, scroll down to Thumbnail Settings. You can set the default thumbnail dimensions and manage older dimension settings.
Keep in mind that these are the maximum thumbnail dimensions that Imagely will use. Some thumbnails may generate slightly smaller if Imagely is not able to create a thumbnail to the maximum dimensions.

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